U.S. patent number 3,958,174 [Application Number 05/568,746] was granted by the patent office on 1976-05-18 for modulated induction generator.
This patent grant is currently assigned to Borg-Warner Corporation. Invention is credited to George Henry Studtmann, Harry James Venema.
United States Patent |
3,958,174 |
Studtmann , et al. |
May 18, 1976 |
Modulated induction generator
Abstract
An induction machine is driven to operate as a generator, and
has its electrical output conductors coupled to a switching system,
which can be an inverter circuit. An oscillator and logic circuit
are connected to regulate the switching of the power switches, such
as thyristors, in the inverter circuit which operates as a
switching system. The thyristors in the switching system are
regulated to switch at a frequency sufficiently below the
synchronous frequency of the induction machine to enable the
machine to build up and operate as a generator. A modulator is
connected to the oscillator to vary the oscillator frequency, and
thus change the switching frequency of the switching system, above
and below a reference level to provide a corresponding a-c output
voltage, with a d-c average level, on the d-c bus conductors of the
inverter circuit which operates as the switching system. The
amplitude and frequency of the a-c output voltage are controllable
over a wide range of variations in both the shaft speed of the
induction machine and/or the electrical load. The system is also
capable of producing an a-c voltage and a separate d-c voltage, and
both the a-c and d-c voltages can be separately controlled.
Inventors: |
Studtmann; George Henry (Mount
Prospect, IL), Venema; Harry James (Wheaton, IL) |
Assignee: |
Borg-Warner Corporation
(Chicago, IL)
|
Family
ID: |
24272559 |
Appl.
No.: |
05/568,746 |
Filed: |
April 16, 1975 |
Current U.S.
Class: |
322/47; 322/94;
322/28 |
Current CPC
Class: |
H02K
17/42 (20130101); H02M 5/27 (20130101); H02M
5/271 (20130101); H02M 7/53875 (20130101); H02P
9/44 (20130101) |
Current International
Class: |
H02M
5/02 (20060101); H02M 5/27 (20060101); H02M
7/5387 (20060101); H02P 9/44 (20060101); H02K
17/42 (20060101); H02P 9/00 (20060101); H02P
009/00 () |
Field of
Search: |
;322/47,28,20,72,94,61
;321/45C,70 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller; J. D.
Assistant Examiner: Hickey; Robert J.
Attorney, Agent or Firm: Jennings, Jr.; James J.
Claims
What is claimed is:
1. A system for providing an alternating output voltage having a
d-c average value, comprising:
an induction machine having a mechanical input shaft for receiving
rotational energy at a first frequency, and having a plurality of
output conductors;
a switching system, comprising an inverter circuit including a
plurality of output connections coupled to the induction machine
output conductors, a pair of bus conductors, a plurality of power
switches, coupled both to the bus conductors and the output
connections, which power switches are connected to receive gating
signals to control their conduction and non-conduction, and a
capacitor coupled between the bus conductors;
a logic circuit connected to apply gating signals to the power
switches as a function of received timing signals;
an oscillator circuit, coupled to the logic circuit, for generating
the timing signals to regulate operation of the switching system at
a second frequency which is less than the first frequency; and
a modulator circuit, coupled to the oscillator circuit, for
applying a modulating signal of given frequency to the oscillator
and modifying operation of the switching system to switch at a
frequency varying above and below the second frequency, thus
providing on the switching system bus conductors a modulated
voltage, the envelope of which varies at the given frequency of the
modulating signal and as a function of the amplitude of the
modulating signal.
2. A system for providing an alternating output voltage having a
d-c average value, comprising:
a multi-phase induction machine having a mechanical input shaft for
receiving rotational energy at a first frequency, and having a
plurality of output conductors;
a switching system, comprising a multi-phase inverter circuit
including a plurality of output connections coupled to the
induction machine output conductors, a pair of bus conductors, and
a plurality of power switches, coupled both to the bus conductors
and the output connections, which power switches are connected to
receive gating signals to control their conduction and
non-conduction and periodically direct the flow of reactive energy
from one phase circuit to another phase circuit of the induction
machine;
a logic circuit connected to apply gating signals to the power
switches as a function of received timimg signals;
an oscillator circuit, coupled to the logic circuit, for generating
the timing signals to regulate operation of the switching system at
a second frequency which is less than the first frequency; and
a modulator circuit, coupled to the oscillator circuit, for
applying a modulating signal of given frequency to the oscillator
and modifying operation of the switching system to switch at a
frequency varying above and below the second frequency, thus
providing on the switching system bus conductors a modulated
voltage, the envelope of which varies at the given frequency of the
modulating signal and as a function of the amplitude of the
modulating signal.
3. A system as claimed in claim 2 and further comprising:
means, coupled between the bus conductors and an a-c load to be
energized by the system, for blocking the d-c component of the
alternating output voltage and passing only an a-c voltage to the
a-c load.
4. A system as claimed in claim 2 and further comprising:
means, coupled between the bus conductors and a d-c load to be
energized by the system, for substantially reducing the a-c
component of the output voltage and passing the filtered d-c
voltage to the d-c load.
5. A system as claimed in claim 2 and further comprising:
a comparator having first and second input connections and an
output connection for providing an output signal which is the
algebraic sum of the input signals received;
means for coupling the comparator output connection to the
oscillator circuit;
means for coupling the first input connection of the comparator to
the modulator circuit to receive the modulating signal; and
means for coupling the second input connection of the comparator to
one of said bus conductors, thus regulating the oscillator circuit
by a signal which represents the difference between the actual
output signal on the bus conductors and the desired signal as
represented by the modulating signal passed from the modulator
circuit to the comparator.
6. A system as claimed in claim 5 and further comprising:
a summing amplifier, having an output connection and a pair of
input connections;
means for coupling the summing amplifier output connection to the
first input connection of the comparator circuit;
means for coupling the first input connection of the summing
amplifier to the modulator circuit to receive the modulating
signal;
means for establishing an adjustable reference d-c voltage signal;
and
means for coupling the second input connection of the summing
amplifier to the means for establishing the adjustable reference
d-c voltage signal, such that the resultant signal passed from the
summing amplifier to the first input connection of the comparator
is adjustable as to d-c level by adjusting the means for
establishing the adjustable reference d-c voltage signal and is
adjustable as to both frequency and amplitude of the modulating
signal by adjustment of the modulator circuit, to provide a
corresponding regulation of the output voltage on the system bus
conductors.
7. A system as claimed in claim 6 and further comprising:
means, coupled between the bus conductors and an a-c load, for
blocking the d-c component of the alternating output voltage and
passing only an a-c voltage to the a-c load, such that the
effective frequency and amplitude of the a-c output voltage can be
regulated by adjustment of the modulating signal provided by the
modulator circuit.
8. A system as claimed in claim 6 and further comprising:
a filter, coupled between the bus conductors and a d-c load, for
effectively minimizing the a-c component of the voltage provided to
the load, such that the effective amplitude of the voltage passed
from the bus conductors to the d-c load can be adjusted by
adjusting the means for establishing the adjustable reference d-c
signal applied to the summing amplifier.
9. A system for providing an alternating output voltage having a
d-c average value, comprising:
an induction machine having a mechanical input shaft for receiving
mechanical rotational energy at a first frequency, and having a
plurality of output conductors;
a switching system, comprising an inverter circuit including a
plurality of output connections coupled to the induction machine
output conductors, a pair of bus conductors, a plurality of
thyristors, coupled both to the bus conductors and the output
connections, which thyristors are connected to receive gating
signals to control their conduction and non-conduction, and a
capacitor coupled between the bus conductors;
a logic circuit connected to apply gating signals to the thyristors
as a function of received timing signals;
an oscillator circuit, coupled to the logic circuit, for generating
the timing signals to regulate operation of the switching system at
a second frequency which is less than the first frequency; and
a modulator circuit, coupled to the oscillator circuit, for
applying a modulating signal of given frequency and amplitude to
the oscillator and modifying operation of the switching system to
switch at a frequency varying above and below the second frequency,
thus providing on the switching system bus conductors a modulated
voltage, the envelope of which varies at the given frequency of the
modulating signal and as a function of the amplitude of the
modulating signal.
10. A system for providing an alternating output voltage having a
d-c average value, comprising:
a three-phase induction machine having a mechanical input shaft for
receiving mechanical rotational energy at a first frequency, and
having a plurality of output conductors;
a switching system, comprising a three-phase inverter circuit
including a plurality of output connections coupled to the
induction machine output conductors, a pair of bus conductors, and
a plurality of thyristors, coupled both to the bus conductors and
the output connections, which thyristors are connected to receive
gating signals to control their conduction and non-conduction and
periodically direct the flow of reactive energy from one phase
circuit to another phase circuit of the induction machine;
a logic circuit connected to apply gating signals to the thyristors
as a function of received timing signals;
an oscillator circuit, coupled to the logic circuit, for generating
the timing signals to regulate operation of the logic circuit and
thus of the switching system at a second frequency which is less
than the first frequency; and
a modulator circuit, coupled to the oscillator circuit, for
applying a modulating signal of given frequency and amplitude to
the oscillator and modifying operation of the switching system to
switch at a frequency varying above and below the second frequency,
thus providing on the switching system bus conductors a modulated
voltage, the envelope of which varies at the given frequency of the
modulating signal and as a function of the amplitude of the
modulating signal.
11. A system as claimed in claim 10 and further comprising:
means, coupled between the bus conductors and an a-c load to be
energized by the system, for blocking the d-c component of the
alternating output voltage and passing only an a-c voltage to the
a-c load.
12. A system as claimed in claim 10 and further comprising:
means, coupled between the bus conductors and a d-c load to be
energized by the system, for substantially reducing the a-c
component of the output voltage and passing the resultant filtered
d-c voltage to the d-c load.
13. A system as claimed in claim 10 and further comprising:
a comparator having first and second input connections and an
output connection for providing an output signal which is the
algebraic sum of the input signals received;
means for coupling the comparator output connection to the
oscillator circuit;
means for coupling the first input connection of the comparator to
the modulator circuit to receive the modulating signal; and
means for coupling the second input connection of the comparator to
one of said bus conductors, thus regulating the oscillator circuit
by a signal which represents the difference between the actual
output signal on the bus conductors and the desired signal as
represented by the modulating signal passed from the modulator
circuit to the comparator.
14. A system as claimed in claim 13 and further comprising:
a summing amplifier, having an output connection and a pair of
input connections;
means for coupling the summing amplifier output connection to the
first input connection of the comparator circuit;
means for coupling the first input connection of the summing
amplifier to the modulator circuit to receive the modulating
signal;
means for establishing an adjustable reference d-c voltage signal;
and
means for coupling the second input connection of the summing
amplifier to the means for establishing the adjustable reference
d-c voltage signal, such that the resultant signal passed from the
summing amplifier to the first input connection of the comparator
is adjustable as to d-c level by adjusting the means for
establishing the adjustable reference d-c voltage signal and is
adjustable as to both frequency and amplitude of the modulating
signal by adjustment of the modulator circuit, to provide a
corresponding regulation of the output voltage on the system bus
conductors.
15. A system as claimed in claim 10 and further comprising:
a capacitor, coupled between the bus conductors and an a-c load,
for blocking the d-c component of the alternating output voltage
and passing only an a-c voltage to the a-c load, such that the
effective frequency and amplitude of the a-c output voltage can be
regulated by adjustment of the modulating signal provided by the
modulator circuit.
16. A system as claimed in claim 10 and further comprising:
a filter, coupled between the bus conductors and a d-c load, for
effectively minimizing the a-c component of the voltage provided to
the load, such that the effective amplitude of the voltage passed
from the bus conductors to the d-c load can be adjusted by
adjusting the means for establishing the adjustable reference d-c
signal applied to the summing amplifier.
Description
BACKGROUND OF THE INVENTION
A general discussion of induction machines is set out in U.S. Pat.
No. 3,829,758, entitled "AC-DC Generating System", which issued to
the assignee of this invention on Aug. 13, 1974. That patent
discloses that the conventional capacitor bank or separate
excitation source for an induction machine can be replaced by a
static inverter circuit which operates not as an inverter but as a
switching system to recirculate the reactive energy. The system of
the earlier patent proved effective in providing an a-c output
voltage on the output conductors of the machine (FIG. 6 of the
patent), and/or a d-c output voltage on the switching system
(inverter circuit) bus conductors. Furthermore it was shown in the
earlier patent that the amplitude of the output voltage, whether
a-c or d-c, is controllable over a wide range of variations in the
shaft speed and/or the electrical load. As taught in that patent,
the output voltage is a function of the slip frequency, which is
the difference between the inverter switching (electrical)
frequency and the machine (synchronous) frequency. By regulating
the slip--or difference between the electrical frequency and
synchronous frequency--the amplitude of the output voltage can be
controlled. However in that system the a-c voltage was basically a
"wild " frequency voltage; that is, the frequency varied with the
synchronous speed of the induction generator. Accordingly it would
be desirable to use such a system to produce an a-c output voltage
which has a constant (or controllable) frequency, notwithstanding
variations in the machine shaft speed and/or the electrical
load.
It is therefore a principal consideration of this invention to
provide such a system with a constant frequency a-c output voltage,
which is maintained constant (or controlled) in spite of
fluctuations in the induction machine shaft speed and/or variations
in the electrical load supplied by the system.
From the subsequent explanation it will become apparent that the
frequency-regulated a-c output voltage has a d-c average value. It
is therefore another important consideration of this invention to
produce such a system in which the a-c output voltage can be
separated from the d-c average voltage, so that both a-c and d-c
output voltages can be provided and independently controlled.
SUMMARY OF THE INVENTION
A system for providing an a-c output voltage constructed in
accordance with this invention comprises a single-phase or a
multi-phase induction machine having a mechanical input shaft for
receiving rotational energy at a first frequency, and having a
plurality of output conductors. A switching system, comprising a
multi-phase inverter circuit, has a plurality of output connections
coupled to the induction machine output conductors, a pair of bus
conductors, and a plurality of power switches, such as thyristors,
coupled both to the bus conductors and to the output connections.
The power switches could also be power transistors, thyratrons, or
other suitable switches known to those skilled in this art. The
power switches are connected to receive gating signals to control
their conduction and non-conduction and periodically direct the
flow of reactive energy from one phase circuit to another phase
circuit of the induction machine. A logic circuit is connected to
apply gating signals to the power switches as a function of rceived
timing signals, and an oscillator circuit is coupled to the logic
circuit, for generating the timing signals to regulate operation of
the switching system at a second frequency which is less than the
first frequency.
Particularly in accordance with this invention, a modulator circuit
is coupled to the oscillator circuit, for applying a modulating
signal of given frequency to the oscillator and modifying operation
of the switching system to switch at a frequency varying above and
below the second frequency. Accordingly this system provides on the
switching system bus conductors a modulated voltage, the envelope
of which varies at the given frequency of the modulating
signal.
In accordance with another aspect of the invention, a comparator
circuit is connected to receive the modulating signal of given
frequency, and another signal which is a function of the output
signal on the bus conductors. The comparator output signal is
applied to the oscillator circuit, to maintain the output voltage
at the desired frequency and amplitude, notwithstanding variations
in the synchronous speed of the machine and/or variations in the
electrical load supplied by the system.
THE DRAWINGS
In the several figures of the drawings, like numerals designate
like components, and in those drawings:
FIGS. 1-4 are graphical illustrations useful in understanding the
invention;
FIG. 5 is a block diagram of a system constructed in accordance
with the invention;
FIG. 6 is a schematic diagram of a circuit shown generally in FIG.
5;
FIG. 7 is a block diagram showing the incorporation of a feedback
loop in the system of FIG. 5; and
FIG. 8 is a block diagram illustrating a circuit for separating the
a-c and d-c components of the output voltage.
GENERAL BACKGROUND DISCUSSION
The teaching of U.S. Pat. No. 3,829,758 is hereby incorporated in,
and made a part of, this disclosure.
Consider initially that the induction machine is rotating at a
mechanical speed which corresponds to a synchronous electrical
frequency of 400 Hertz (Hz.). If the actual electrical frequency is
sufficiently less than 400 Hz., then the machine will operate as a
generator. FIG. 1 depicts in curve 10 the variation of generator
output voltage as a function of electrical frequency, given a
constant load. As the electrical frequency is gradually reduced
from 400 Hz., generator action will commence at a point designated
A. The difference between the synchronous frequency of 400 Hz. and
the lower frequency at point A is a function of the load, or an
inverse function of the load resistance.
Assume, for purposes of explanation, that it is desired to operate
the system at an electrical frequency of 388 Hz. and a voltage
amplitude of 200 volts. This corresponds to operation at point B on
curve 10. Now if the electrical frequency is varied as a function
of time, so that it is sometimes greater and sometimes less than
the reference frequency of 388 Hz., FIG. 2 indicates what will
happen to the output voltage (as seen on the bus conductors of the
switching system) as the electrical frequency swings above and
below 388 Hz. The varying or modulating voltage is represented by
curve 11, and the output voltage by curve 12. The output voltage
attempts to follow the modulating voltage, so that the output
voltage varies above and below 200 volts. Thus by modulation of the
electrical frequency, a modulated output voltage can be produced as
shown in FIG. 2.
If the machine speed is doubled so that the synchronous frequency
is now 800 Hz., the same modulation technique can be applied as
shown in FIG. 3. As there indicated curve 13 represents operation
with the machine generating, and the distance between point C and
the 800 Hz. mark represents the load losses. In this system the
reference frequency is 776 Hz., so that the reference point occurs
at D on curve 13. The modulating frequency is set to swing from
approximately 768 to 784 Hz., as shown by curve 14. The result is a
modulated output voltage as shown in curve 15. It is noted that the
output voltage has a frequency which depends only on the frequency
of the modulating voltage shown in curve 14. The frequency of the
output voltage shown in curve 15 is independent of the induction
machine speed.
The change in machine speed does affect the shape of the
voltage/frequency curve shown in FIG. 1. However, several different
factors are involved in the modification of the curve shape, and as
a first order approximation the curve shape can be considered
constant for different operating frequencies.
It will be apparent from inspection of FIGS. 2 and 3 that the
amplitude and frequency of the output voltage represented by curve
12 are direct functions of the modulating signal represented by
curve 11. That is, controlling the amplitude of the modulating
signal 11 will correspondingly control the amplitude of the output
voltage 12. Similarly a change in the frequency of modulating
signal 11 will produce a corresponding change in the frequency of
output voltage 12.
From the above it is apparent that the system has the capability of
maintaining both the frequency and the amplitude of the a-c voltage
envelope at desired values when the mechanical speed of the
induction generator varies.
Considering load changes, if the synchronous frequency is
maintained at 400 Hz. but the load is, say, doubled (that is, the
resistance is halved), the voltage/frequency curve is changed as
shown in FIG. 4. The initial generator build-up point now occurs at
a lower frequency, represented by point E. In general the slip at
the point where generator build-up first begins is inversely
proportional to the load resistance. In addition, the maximum
output voltage of the machine is somewhat less for the heavier
load.
If it is now required to maintain the output voltage represented by
curve 17 at the same amplitude, the modulation is adjusted to cause
the frequency to vary about point F on curve 18. The output voltage
swing would then remain bounded by the dashed lines 20, 21. It is
apparent, therefore, from the foregoing explanation that it is
possible to regulate the modulated output voltage of the system
both variation frequency and amplitude by the technique described
above, notwithstanding variations in the shaft speed of the machine
and/or changes in the electrical load.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 5 depicts in a block arrangement the basic sub-systems of the
present invention. An induction machine 25 has an input shaft 26 to
receive mechanical drive for rotating the machine at a first or
synchronous frequency. This corresponds to the synchronous
frequencies shown in the drawings, such as 400 Hertz referenced in
FIG. 1. The electrical output connections of the machine are
coupled over conductors 27, 28 and 29 to a switching system 30
which can be an inverter circuit, as specifically illustrated in
FIG. 7 of U.S. Pat. No. 3,829,758. The inverter circuit has a pair
of bus conductors 31 and 32, which correspond to conductors 60, 61
in FIG. 7 of the reference patent. In addition an excitation system
shown as switch 33 and battery 34 is provided to assist the system
in the initial build-up. This corresponds to the circuit 16 in FIG.
7 of the reference patent and, like that circuit, can be
disconnected after the system has been started. A capacitor 35 is
coupled between bus conductors 31 and 32. Six power switches, shown
as mechanical switches 36-41, are provided to conduct or interrupt
the flow of current in response to receipt of turn-on (gating)
signals or turn-off (commutation) circuit operation. The power
switches can be thyristors, power transistors, vacuum tubes, or any
appropriate switch. The turn-on and turn-off of the power switches
within the switching system are regulated by signals generated in
logic circuit 42 and applied over line 43 to the switching system
30. such a logic circuit can, by way of example, be of the "Three
Stage SCR Ring Counter Circuit" type shown in FIG. 7.17 on page 109
of G.E.'s SCR manual second edition. In turn the logic circuit
operation is determined by timing signals received over line 44
from an oscillator circuit 45. One suitable example of the
oscillator arrangement will be set out in more detail in FIG. 6,
but those skilled in the art will appreciate that various types of
oscillators can be used, so long as an appropriate timing pulse is
supplied to the logic circuit.
Particularly in accordance with the present invention, a modulator
circuit 46 is provided and coupled over line 47 to the oscillator
45 to vary the timing of the pulses produced by the oscillator
circuit. The modulator unit can be any of many different
commercially available types, for example, such as the
Hewlett-Packard Model No. 202A Function Generator. In relating the
operation of the system of FIG. 5 to the showing of FIG. 2 in this
disclosure, the induction machine 25 is rotated at a first (or
synchronous) speed by the input mechanical energy. This synchronous
frequency is that referenced 400 Hertz in FIG. 2. The electrical
frequency of operation is established by timing the switching of
the power switches in the switching system (inverter circuit) 30.
This timing is accomplished by the frequency of the pulses produced
in oscillator 45, in turn modulated by the input signal from
modulator 46. This modulating signal is represented by the curve 11
in FIG. 2, where the electrical frequency is shown to vary above
and below 388 Hertz as a function of the modulating signal applied.
Without any modulation, the switching frequency would be kept
constant at the reference frequency of 388 Hertz, and the output
voltage on the bus conductors of switching system 30 would be a
constant amplitude 200 volts d-c signal. With modulation, there is
a swing or variation in output voltage to provide in effect an a-c
signal on the bus conductors of the switching system. Of course the
d-c component of the output voltage can be blocked by a
series-connected capacitor in a manner known to those skilled in
the art, so that only the a-c signal represented by curve 12 in
FIG. 2 is provided on the output to the associated equipment. Thus
the system of FIG. 5 provides a constant frequency on the output
bus conductors.
FIG. 6 shows one circuit suitable for use as the oscillator 45 of
the system shown in FIG. 5. In FIG. 6 the various components are
shown with their specific identifications of transistor types,
resistor values and so forth, to afford implementation of the
invention with a minimum of experimentation. If desired a general
background operating description of such an oscillator circuit is
set out in U.S. Pat. No. 3,406,355, entitled "Oscillator Circuit",
which issued to the assignee of this invention on Oct. 15, 1968. In
FIG. 6 the modulator 46 provides a signal over an output impedance
referenced Z.sub.46 to the oscillator circuit to control the
oscillator frequency. The oscillator was designed to operate with a
voltage of 15 volts positive applied to conductor 48, with respect
to the potential on the ground conductor 49. The output signal is
provided over the two conductors referenced 44a and 44b for
application to the logic circuit.
The system described thus far has all the components required to
produce an output voltage which is controllable both in frequency
and amplitude, as a function of the modulating signal applied to
the oscillator. Those skilled in the art will appreciate that a
feedback loop can be added to the system shown in FIG. 5 to provide
automatic, continuous regulation of the output voltage. FIG. 7
depicts such a system with the incorporation of a feedback
loop.
As there illustrated the system is generally similar to that shown
in FIG. 5, but a feedback loop including a comparator 50 is
incorporated. The output voltage (or a portion of this voltage) is
applied over line 51 to one input connection of comparator 50. The
output of modulator 46, acting as a reference signal, is applied
over line 52 to the other input connection of comparator 50.
Accordingly the output voltage is compared to a reference voltage
in comparator 50, and the resultant difference signal is applied
over line 53 to control the oscillator 45. If now the reference
voltage on line 52 is amplitude modulated at some relatively low
frequency, then the output voltage will be conformed with the
reference voltage by nature of the feedback system. Thus any
variations of the machine speed and/or the load impedance will be
compensated for by the feedback system.
As will be evident from examination of the various wave shapes,
particularly wave shape 12 of FIGS. 2 and 3, the output voltage has
both a d-c component and an a-c component. In order to obtain pure
a-c, the d-c component must be separated or blocked, which can be
done with a capacitor as is known to those skilled in the art. To
obtain the pure d-c component, a low-pass filter, such as an LC
filter, can be used to produce the d-c as well as the a-c
component. This is illustrated more clearly in FIG. 8
As there is shown, both an a-c load 55 and a d-c load 56 are
provided. A blocking capacitor 57 is coupled between the bus
conductors 31, 32 and the a-c load 55, to block the d-c current. An
LC filter including a series-connected inductor 58 and a
parallel-connected capacitor 60 is coupled between the bus
conductors and d-c load 56.
Moreover the amplitudes of the a-c output voltage and the amplitude
of the d-c output voltage can be independently controlled by the
arrangement shown in FIG. 8. To this end a summing amplifier 61 is
connected to receive both the modulating a-c signal 62, over line
52 from modulator 46, and an adjustable amplitude d-c reference
signal 63, over line 64 from potentiometer 65. Thus the output
signal 66 of the summing amplifier, having both a-c and d-c
components, is applied over line 67 to comparator 50. The
comparator 50 receives another input signal, representing both the
a-c and d-c components of the system output voltage, over line 51.
Thus the oscillator circuit 45 is controlled as a function of the
difference between the actual system output voltage and the a-c and
d-c signals established by modulator 46 and potentiometer 65. By
varying, for instance, the d-c level set by potentiometer 65, the
d-c output level of the summing stage 61 would shift but the a-c
variation would not. This would then cause the oscillator frequency
to be modulated in such a manner as to produce a corresponding
output on the output bus conductors 31, 32. Similarly, if only the
a-c signal 62 is changed in frequency or amplitude, the d-c signal
level is held constant and summing amplifier 61 produces a summed
reference signal with the a-c portion appropriately varied. The
summing amplitude output is then compared with the system output
voltage and appropriate corrections made until the system output
has that same wave shape.
From the above, it is apparent that regulation of the system output
voltage can be maintained independently of variations in the
voltage on the output bus. Those skilled in the art will appreciate
that the current sensing arrangement shown and described in the
reference patent, particularly in FIG. 9 can be used in connection
with this invention, to provide current limiting operation.
It is noted that the amplitude of the modulating voltage is always
such, as shown for example in FIG. 2, that the swing or variation
along the curve 10 does not extend to the right of the point A,
where the curve 10 intersects the frequency axis. This is the point
at which, with an increasing frequency, generating action would
cease. If the operating frequency were thus increased to the point
A where the induction machine was no longer generating, it would
have to build up again when the frequency was again decreased to
the initial build-up at point A. This would result in distortion of
the resultant output voltage curve. To avoid this distortion, the
amplitude of the modulating voltage is maintained such that a
suitable range of the curve 10 is used and there is never any
extension of the modulating signal into the region to the right of
point A.
At this time the best mode known for practicing the invention is
shown in FIG. 5, with modulator 46 connected to provide effective
modulation of the signal produced by oscillator circuit 45.
Additional refinements can be incorporated, as described in
connection with FIGS. 7 and 8, but the basic system of FIG. 5 is
employed in conjunction with these other embodiments.
In the appended claims the term "connected" means a d-c connection
between two components with virtually zero d-c resistance between
those components. The term "coupled" indicates there is a
functional relationship between two components, with a possible
interposition of other elements between the two components
described as "coupled " and "inter-coupled".
While only particular embodiments of the invention have been
described and claimed herein, it is apparent that various
modifications and alterations of the invention may be made. It is
therefore the intention in the appended claims to cover all such
modifications and alterations as may fall within the true spirit
and scope of the invention.
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